Lots of options and plans out there for “simple” benches. Mr. Sellers has a nifty series underway on YouTube for a simple bench design.
My three thoughts are:
1) Anything is better than nothing. Workmates aren’t terrible, especially if you also invest in some sandbags to weigh down the base.
2) You will build (or buy) more than one bench as you progress. Size, features, project requirements, any number of reasons to update or add-on.
3) Sturdy trumps clever. Make (or buy) it heavy so you aren’t spending time and energy chasing it around the room and you will get much more work done with less effort.
Fancy vices are nice but I seem to do more work with battens, wedges, dogs and holdfasts (gravity & friction are your friends) even with nice end and face vices. A good friend of mine made a small portable bench from a linden log he harvested and it uses all wedges and dogs for the workholding. Works very well and knocks down as it is all mortise and tusk-pinned through tenons.